Speciation and crystal chemistry of Fe(III) chloride hydrolyzed in the presence of SiO4 ligands. 2. Characterization of Si-Fe aggregates by FTIR and Si-29 solid-state NMR

This work aims at a better understanding of the interactions existing betwe en Fe and Si in freshly precipitated Fe(III)/Si phases (prepared at pH 3, 5 , 7, and 10 with Si/Fe molar ratios in the range 0.25-4). By coupling the r esults of two spectroscopic techniques, FTIR and Si-29 NMR, interesting str uctural information emerges. We show that Si and Fe atoms do not form separ ate silica and FeOOH particles and that the presence of Si-O-Fe bonds hinde rs the formation of Fe oxyhydroxides. The ratio Si/Fe = 1 constitutes a tra nsition point between Si-O-Fe and Si-O-Si bond formation at pH = 3 and 5, w here Si-clusters appear once the maximum amount of Si-O-Fe bonds are formed . This is confirmed by Si-29 NMR which demonstrates the presence of Si pock ets in three of the eight examined samples. The return to equilibrium of th e Si-29 magnetization leads to a value for both dimensionality of the silic a-rich pockets to = 2.2) and length over which dimensionality is observed ( 2 nm). By using both FTIR and Si-29 solid-state NMR, we clearly demonstrate how the pH of synthesis determines the structural properties of the formed samples. The results obtained are in good agreement with our previous stud y conducted by EXAFS.